High Pressure ResearchVol. 29, No. 2, June 2009, 254–260

The A(II)Cr(IV)O3 (A = Sr, Ca, Pb) ‘simple’ perovskites.Structure and properties: magnetic structure of CaCrO3

†

Miguel Á. Alario-Franco*, Elizabeth Castillo-Martínez and Ángel M. Arévalo-López

Facultad CC. Químicas, Departmento de Química Inorgánica I, UCM, Madrid, Spain

(Received 5 September 2008; final version received 11 November 2008 )

Dedicated to Professor H. Tracy Hall, who designed the belt-type high-pressure chamber.

The ‘simple’ perovskites ACr(IV)O3 (A = Sr, Ca, Pb) have been synthesized at high pressure and tem-perature. Their microstructure and properties are discussed. Specific heat and susceptibility measurementsindicate an antiferromagnetic ordering at low temperature in CaCrO3, which is confirmed to be of theC-type by neutron diffraction.

Keywords: perovskite; CaCrO3; high pressure

1. Introduction

It is known that Cr(IV) in octahedral coordination requires high-pressure conditions to be obtained.In particular, it has been reported that SrCrO3 is stabilized at about 6.5 GPa and 1023 K [1], while6 GPa and 973 K have been used for obtaining CaCrO3 [2] and 6.5 GPa was used to obtainPbCrO3 [3]. These materials, already synthesized in the 1960s, have been the subject of recentworks [4–6].

SrCrO3 is a cubic perovskite, initially reported to be a metallic Pauli paramagnet [1], andrecently found to present phase segregation to a phase of tetragonal symmetry with a C-type antiferromagnetic (AFM) ordering at low temperature [6]. Polycrystalline samples showa semiconducting behavior [7], that under pressure turns into metallic [4].

CaCrO3 is an orthorhombic perovskite which was initially described as an AFM belowTN = 90 K [2], and whose electrical properties also present the same controversy, dependingon whether a single crystal (metallic) or a powdered sample (semiconducting) are measured. Thesame transition to metallic occurs in this powdered sample under pressure [4].

We have prepared and characterized the novel solid solution Sr1−xCaxCrO3(0 < x < 1); thedifferent members can be obtained at pressures higher than 4 GPa and temperatures between 973

*Corresponding author. Email: maaf@quim.ucm.es†This paper was presented at the XLVIth European High Pressure Research Group (EHPRG 46) Meeting,Valencia (Spain),7–12 September, 2008.

ISSN 0895-7959 print/ISSN 1477-2299 online© 2009 Taylor & FrancisDOI: 10.1080/08957950802618365http://www.informaworld.com

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and 1323 K depending on the calcium content. They show a progressive structural evolution alongdifferent modifications of the perovskite structure [8]. Together with the structural changes, there isan increase in the temperature at which magnetic interactions start as calcium substitutes strontium,as well as an increase in the electrical resistance, with a maximum observed for CaCrO3 [9].

PbCrO3 for a long time has been reputed to be a simple insulating cubic perovskite [10]. We haverecently shown that it has a very complicated compositionally modulated perovskite superstruc-ture [11]. We will report in here some of our recent results concerning the magnetic properties andspecific heat measurements of the ‘simple’ACr(IV)O3 perovskites, including the first descriptionof the magnetic structure of CaCrO3 as determined from powder neutron diffraction data obtainedat low temperature.

2. Experimental

Stoichiometric amounts of the starting materials, SrO, CaO, PbO and CrO2, are mixed and thenintroduced into gold or platinum capsules for high-pressure synthesis. When SrO or CaO are used,they are handled in an inert atmosphere to avoid reaction with H2O and CO2; the capsules areclosed inside a glove box.

The synthesis experiments were performed in a ‘belt-type’ apparatus at the Laboratorio Com-plutense de Altas Presiones (http://www.ucm.es/info/labcoap/index.htm) [8,11]. We have usedour highest achievable pressure, 8 GPa, in order to obtain PbCrO3. Experiments performed athigher pressures at the Bayerisches Geoinstitute have shown that at 1273 K it becomes amorphousat pressures above 10 GPa.

All of the samples have been characterized by means of X-ray powder diffraction performedin a Panalytical X’Pert PRO ALPHA1 (CuKα1 source, Ge monochromator) diffractometer. Thediffraction patterns were refined by the Rietveld method using the Fullprof program.

15% of β-CaCr2O4 is present in CaCrO3; however, measurements on the pure phase show thatit is not responsible for any of the features found in the magnetic or heat capacity measurements.

DC magnetic susceptibility and Mvs. H were measured on a MPMS-SQUID magnetometer(Quantum Design, MPMS) between 2 and 300 K, in applied fields of 1 T under both zero-field-cooled (ZFC) and field-cooled (FC) conditions.

Specific heat, Cp, was recorded at zero static magnetic field between 2 and 300 K on heatingby a pulse relaxation method using a commercial calorimeter (Quantum Design PPMS).

Neutron powder diffraction (NPD) data were collected in the D20 instrument, equipped witha cryostat, at the Institute Laue-Langevin (ILL) [12]. The diffractometer was used in both highresolution (HR) (λ = 1.36 Å, 2θstep = 0.1◦) for the nuclear structure, and high flux (HF) mode(λ = 2.42 Å, 2θ step = 0.1◦) for the magnetic one. One hundred milligrams of a powdered sample(CaCrO3) were introduced in a 5 mm vanadium can. Data were collected for 3 h at each temperaturein the HF mode and for 6 h in the HR mode. An additional measurement was carried out at eachmode for cryostat background subtraction, which becomes very significant for such a small amountof sample.

3. Results and discussion

3.1. Magnetic susceptibility measurements

The temperature dependence of the magnetic susceptibility at 1 T for the three perovskites isshown in Figure 1 (left), and their inverse in Figure 1 (right). All three show a deviation fromlinearity in the inverse of the susceptibility. Above 150 K and 90 K the data for the SrCrO3 and

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Figure 1. (Left) ZFC and FC dc magnetic susceptibility curves of Sr/Ca/PbCrO3 measured at 1 T. (Right) Inversemagnetic susceptibility curves.

CaCrO3 can be fitted to a Curie–Weiss (C–W) law, and the calculated effective magnetic momentsare 2.83 μB and 3.6 μB, respectively. For PbCrO3 a deviation from linearity in the inverse of thesusceptibility is evidenced up to 300 K; therefore it can not be adjusted for a C–W law.

It is interesting to note that in our measurements, we were not able to observe the reportedAFM order at 50 K for the SrCrO3, but small anomalies were observed at lower fields. However,a deviation at low temperature is appreciated.

Moreover, CaCrO3 presents a clear FC–ZFC splitting; this is due to anAFM ordering with someweak ferromagnetic (WFM) component in the spins, as the neutron data confirm. Besides this,PbCrO3 presents another transition at lower temperature, ≈30 K; the possible magnetic orderingin such a complex structure would presumably present a glassy behavior, but this would requirea further study.

3.2. Heat capacity

The molar heat capacity against temperature, Cp vs. T , of the samples Sr/Ca/PbCrO3 is shownin Figure 2 (left), with Cp/T vs. T represented in the inset. Despite the previous report of aphase segregation and the 3D-AFM order in SrCrO3 at low temperature [6], no clear anomaly isobserved for this compound. On the other hand, an anomaly is observed for CaCrO3, indicatingthe existence of the magnetic transition suggested by susceptibility measurements [13] and nowconfirmed by specific heat and neutron data.

In spite of the differences in crystal symmetry, S.G. Pm–3m for SrCrO3, and the orthorhombicdistortion due to octahedral tilting, S.G. Pbnm for CaCrO3, their curves almost lie on top of eachother in the whole temperature range. But, compared with PbCrO3, a large difference is observed.The origin of this large difference may be related to the complex compositionally modulatedsuperstructure observed on this perovskite [11]. No other compound has ever been reported topossess a structure such as that of the high pressure obtained PbCrO3, and therefore no phase canbe used to compare the possible existence of a contribution of the magnetic ordering to the specificheat curve. However, the fact that the signal of PbCrO3 decreases at high temperature could beindicative of the phonon + electronic specific heat of the structure being lower, and the highervalue being due to the glassy magnetic ordering that seems to happen in a very wide temperaturerange.

In the Cp vs. T curve (Figure 2 (left)), the three systems approach, at room temperature,the classical Dulong-Petit result for a system containing 5 atoms per unit formula, Cp ≈ 15R ≈120 J/mol K at 300 K.

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Figure 2. (Left) Cp vs. T curves between 2 and 300 K for Sr/Ca/PbCrO3 (inset shows Cp/T vs.T ). (Right) Cp/T vs.T 2

in the low-temperature range.

In the calcium sample, the jump in the heat capacity occurs at the temperature of the magneticphase transition. Subtracting the lattice and electronic contributions for CaCrO3, by fitting theCp vs. T curves to a fifth-order polynomial (except for the data around the transition), makes itpossible to calculate the heat (enthalpy) of the transition (90 J/mol). The entropy associated withthe magnetic ordering is calculated as Sm = ∫

(Cm/T )dT and it results in Sm = 1.12 J/mol K;this is much smaller than the theoretical spin-only value of R ln(2S + 1) = R ln 3 = 9.13 J/K molexpected for such a system. This value, however, is comparable to that found for theAFM orderingin other d2 perovskites, such as YVO3 [14].

In Figure 2 (right), Cp/T vs. T 2 is shown in the low-temperature range. The three signalscan be fitted to a linear relation of the Debye law Cp/T = γ + β1T

2 with γ the Sommerfeldconstant and β1 = 12π4kBN/5θ3

D with θD the Debye temperature. Their corresponding valuesbeing γ = 18 and 32 mJ/mol K for Sr and Ca, respectively, and θD = 229, 226 and 114 K for Sr,Ca and Pb perovskites, respectively. The lower value of θD found for the Pb perovskite agreeswith its higher molecular weight.

3.3. Neutron diffraction study of CaCrO3

3.3.1. Room and low-temperature structure of CaCrO3

The crystal structure of CaCrO3 has been determined by neutron diffraction data collected at298, 120 and 1.5 K in the D20 diffractometer at ILL (HR, λ = 1.359Å). Neutron diffractionpatterns have been refined with the Pbnm space group for all temperatures, taking into accountthe presence of the impurity β-CaCr2O4 [15]. The evolution of the refined lattice parameters canbe seen in Figure 3 (left). As a result of the usual thermal lattice contraction, lattice parametersare smaller at 120 K than at room temperature; however there is an increase in the basal latticeparameters (ab-plane) on cooling from 120 to 1.5 K, with a more pronounced decrease in the c

parameter and a total volume reduction. This is indicative of a structural distortion and in factit was already pointed out, in earlier work [13], that a transition from an O-orthorhombic to anO ′-orthorhombic distorted perovskite could take place, characteristic of spin-orbit coupling andlocalized d electrons having collinear spins [2].

It can be observed that at 298 and 120 K, lattice parameters follow the sequence a < c/√

2 < b,which corresponds to the O-type structure, whereas at temperatures below the magnetic ordering(TN = 90 K) it changes to a distorted modification O ′ with lattice parameters c/

√2 < a < b. An

abrupt change in the volume is also observed at the phase transition. CaCrO3 is the only case knownto date of a Cr4+ orthorhombic perovskite; yet the octahedral metal Cr(IV) 3d orbital configuration:

258 M.Á. Alario-Franco et al.

Figure 3. (Left) Thermal dependence of lattice parameters a, b and c/√

2 of CaCrO3. (Right) 〈Cr–O〉 distances. Errorbars correspond to the standard deviation as obtained from the refinement. (Lines are guides to the eye).

t22ge

0g is the same as that of V3+ present in the widely studied rare earth orthovanadates (REVO3).

In this family, such a distortion has been observed to be often accompanied by an antiferrodistorsivetype, Q2, orbital ordering (OO) as a consequence of a Jahn–Teller (JT) distortion [14].

In the case of CaCrO3 at room temperature, X-ray data seem to indicate that a Q2-type OOtakes place; however, the neutron data show that the [Cr–O2] and the short [Cr–O1] distances arenot different within their standard uncertainties, and so this OO cannot be proved with the presentdata. Moreover, neutron data collected at lower temperature show that the octahedra become moreregular in the ab-plane but compressed in the axial direction, [Cr–O2].

This situation is similar to that found in the low-temperature tetragonal phase (P 4/mmm) ofSrCrO3, where there is a partial OO with one localized electron in dxy and the other one degeneratedbetween dxz and dyz, which results in a c/a < 1 and a compressed octahedron with four long andtwo short [Cr–O] distances [6]. In CaCrO3, however, the octahedral tilting is responsible for the

Figure 4. Rietveld refinement of the neutron diffraction patterns of CaCrO3 collected at 150 K (left) and 4.2 K (right)with λ = 2.42 Å.

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Figure 5. Crystal structure (left) and magnetic structure (right) of CaCrO3 at 4.2 K. Magnetic moments lie in theab-plane. Blue circles represent chromium atoms, green balls are calcium. Oxygen atoms occupy the vertex of theoctahedra and have been omitted for clarity on the right model (color online).

increase in spin canting as it is observed by the larger splitting between FC and ZFC curves at lowtemperature compared to tetragonal SrCrO3. Moreover, a tetragonal distortion is also found in thebinary Cr(IV) oxide, CrO2. But, in this compound that crystallizes in the rutile structure, the Cr4+is in the center of an elongated octahedral with two long and four short [Cr–O] distances [16],however the octahedral connectivity is quite different in this compound.

3.3.2. Magnetic structure of CaCrO3

The low-temperature magnetic structure of CaCrO3 has been solved by means of neutron diffrac-tion data collected in the HF mode at 4.2 K. Two new reflections appear at temperatures between50 and 150 K, due to a 3D AFM ordering (Figure 4). At 4.2 K the magnetic moments on theCr atoms are antiferromagnetically aligned in the ab-plane with a zero component along thec-axis; this corresponds to the (Fx ,Cy ,0) or (Cx ,Fy ,0) spin configurations (Figure 5 (right)).Refinement with the spins aligned along the b-axis resulted in better agreement factors, withRM = 14.6%, whereas with the spins aligned along the a-axis, RM = 24%, with magneticmoments of Mtotal = 1.00(5) μB and Mtotal = 0.96(5) μB, respectively.

4. Conclusions

The study of the known ‘simple’AIICrIVO3 perovskites shows that they are substantially different.The only simple perovskite structure appears to be SrCrO3 (cf. SrTiO3), which is a semiconductorshowing some magnetic interactions at low temperature but not a well-defined magnetic ordering.

In the case of CaCrO3 at room temperature, X-ray data seem to indicate that a Q2-type OOtakes place, with the axial [Cr–O2] distance intermediate between the two [Cr–O1] distances;however, the neutron data show that the [Cr–O2] and the short [Cr–O1] distances are not differentwithin their standard uncertainties, so this OO cannot be proved with the present data. In any case,neutron data show that at lower temperatures, the octahedra become more regular in the ab-plane,with both [Cr–O1] distances equal, but compressed in the axial direction, [Cr–O2], similar thatof tetragonal SrCrO3. Also, its low-temperature magnetic structure has been determined: it is

260 M.Á. Alario-Franco et al.

orthorhombic at 1.5 K and can be described as (0, Cy, 0) with 1 μB antiferromagnetically orderedparallel to the b-axis, with successive [0 0 1] planes ferromagnetically aligned, and a small cantingalong the a-axis.

PbCrO3 is the most complicated of the three, presenting a complex microstructure. It showstwo clear anomalies in the magnetic susceptibility data at ≈ 180 K and ≈ 30 K.

Heat capacity measurements on the Sr/Ca/PbCrO3 perovskites show that there is an anomalouscontribution in PbCrO3.

Acknowledgements

Samples were obtained at LABCOAP (http://www.ucm.es/info/labcoap/index.htm) with the help of Dr Jose ManuelGallardo Amores. The authors want to acknowledge Dr Paul Henry and Dr Joost van Duijn for helping with the neutrons.The ILL is also acknowledged for providing beam time and financial support for the neutron experiments. High-pressureexperiments (> 8 GPa) were performed at the Bayerisches Geoinstitute under the EU Research Infrastructures: Transna-tional Access Programme (Contract No. 505320 (RITA)-High Pressure). A.M.A.-L. acknowledges CONACYT for aPh.D. grant. This work was funded by CICYT (Project MAT2007-64007), Comunidad Autónoma de Madrid (ProgramMATERYENER, PRICYT S-0505/PPQ-0093,2006), and FundaciónAreces (Ayudas 2004): Física de Bajas Temperaturas.

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